1. Field of the Invention
This invention relates to carbon-carbon composites and, more specifically, to carbon-carbon composites utilizing a filler powder to oxidize and seal cracks formed in the composites due to thermal expansion differences between the carbon-carbon and coating during high temperature thermal cycling.
2. Brief Description of the Prior Art
Carbon-carbon technology is well known in various art areas, primarily in aerospace technology, for its ability to provide structures therefrom which display high strength relative to the structural weight. Carbon-carbon composites incorporate carbon (including graphite) fibers, which may be in the form of a cloth, in a carbon matrix to reinforce the matrix. Carbon-carbon composites are formed, for example, by impregnating carbon (including graphite) cloth with a carbon char-yielding material, such as, for example, a resin, layering plies of the impregnated carbon cloth one atop the other and then carbonizing the impregnated layered plies with the application of heat and pressure. A problem with such carbon-carbon structures is that, when subjected to high temperatures where appropriate oxidizing conditions exist, they have a tendency to oxidize. Furthermore, when such structures are subjected to thermal cycling, especially with high temperatures and large temperature gradients, small cracks may appear in the structures themselves or in any coatings applied thereto.
Current state of the art carbon-carbon composites display a lessening of this problem and have been rendered substantially more oxidation resistant by providing powders distributed throughout the carbon matrix to getter oxygen, form a sealant glass phase and seal cracks in an externally applied coating. An example of such prior art is provided in Ser. No. 06/805,887, filed Dec. 5, 1985 now U.S. Pat. No 4,892,790 and Ser. No. 07/081,126, filed Aug. 3, 1987 of Gray et al., now U.S. Pat. No. 4,795,677 both of which are continuations-in-part of Ser. No. 06/798,994, filed Nov. 18, 1985 now abandoned, which is a continuation in part of Ser. No. 06/676,985, filed Nov. 30, 1984, now abandoned, all of which are incorporated herein in their entirety by reference. These cracks are caused by a thermal expansion difference which exists between the carbon fibers, carbon cloth or substrate which have a relatively low coefficient of thermal expansion and the coating or matrix which has a relatively high coefficient of thermal expansion.
The sealant glasses which form due to oxidation of these powders or fillers are typically multi-component borate glasses. Therefore, in addition to some boride or boron, the composites contain other particulate fillers which provide the necessary ingredients to form the correct composition glass sealant. For example, within the TiO.sub.2 -SiO.sub.2 -B.sub.2 O.sub.3 system, the B.sub.2 O.sub.3 is mainly supplied by elemental boron and the TiO.sub.2 and SiO.sub.2 by combinations of SiC, Ti.sub.5 Si.sub.3 and TiB.sub.2. These fillers are added to the resin prior to prepregging the carbon or graphite cloth.
Composites made from multicomponent powder mixtures of the prior art as described above contain all of the essential ingredients for effective oxidation protection from a macroscopic point of view. However, from a microscopic point of view, the constituent powders are usually segregated. For this reason, during oxidation, these powders oxidize preferentially, forming a glass sealant which is heterogeneous or segregated on a microscopic scale.